The present invention relates generally to the field of telecommunications and, more particularly, to data communications over telephone networks and more specifically to voice band modems operating at data signaling rates of up to 33600 bits/second for use on telephone switched networks and leased point-to-point two-wire telephone type circuits in which a method for improving noise immunity using a 64 state four-dimensional (4D) Trellis coder in the DMT (discrete multi-tone) physical layer.
The explosive growth of internet has created a demand for high data rates for home users that rely on standard analog plain old telephone systems (POTS) that use a copper wire twisted pair to carry the information. The need for high-speed access to the home is inevitable due to the availability of information, data, high-bandwidth video and the like from the world wide web and because of such demand, higher speed modems are required. Modems operating at higher rates of up to 33,600 bits/second are in use nowadays. A multitude of competing communication technologies provide high-speed access to the home such as cable modems, digital subscriber line (xDSL) that utilizes the existing analog plain old telephone systems (POTS) that use a copper wire twisted pair to carry the information. Because of bandwidth limitation (4 KHz), and power limitation of the telephone network, line coding schemes are used to encode digital signals into analog signals that convey the analog information over the analog telephone network, such line coding schemes should avoid undesirable bandwidth or power increase. The line coding schemes manipulate the analog carrier signal which has three attributes (amplitude, phase and frequency) of which one or more of such attributes are manipulated by known modulation techniques, one of which is quadrature amplitude modulation (QAM) whereby the carrier signal's phase and amplitude is modulated in order to encode more data within a frequency bandwidth. One example of a QAM modulation system sends two bits of information per QAM symbol, where the digital values can be encoded and the corresponding amplitude and phase can be represented using the constellation. By increasing the constellation size and in the meantime the bit density per symbol will be increased, hence achieving higher data rates. As the constellation size increases, the granularity of the phase and the amplitude difference between different constellation points diminishes, making it increasingly difficult to decode the constellation points, especially in the presence of noise. One way of circumventing such a problem is to increase the Euclidean distance between symbols employing Trellis coding which is a bandwidth efficient, since the symbol rate and, hence the bandwidth is not increased. As the constellation size gets bigger, the problem of detecting constellation due to density increases and shorter Euclidean distance between symbols, therefore it is desirable to introduce redundancy without doubling the constellation size. Therefore, a way of counter-acting the effects the short Euclidean distance between symbols is to partition the quadrature amplitude modulated signal into subsets, thereby creating an acceptable Euclidean distance between symbols.
Data from a personal computer or other equipment at the customer premise (CPE) is sent to a transmitter which arranges the data into frame packets; the packetized signal is then quadrature amplitude modulation encoded and error encoded using trellis encoding to improve the noise immunity using a convolutional coder to select a sequence of subsets in a partitioned signal constellation. A numerical symbol vector is trellis encoded. The trellis encoding starts with the most significant symbol and ends with the least significant symbol of the vector, a process which employs convolutional encoding that converts the input symbol to another symbol and then maps the encoded symbol to its corresponding 16 QAM signal constellation point. The current trellis encoder in the DMT (discrete multi tone) standard is a 4-D (four-dimensional), 16-state Wei encoder which is a ¾ encoder with a coding gain of 4.6 dB (it varies depending on the bits per bin). The 4-D symbol is formed by 2 bins, each of which contains 2-D (two-dimensional) constellations partitioned into four 2-D co-sets which are further partitioned into 8 4-D co-sets. The minimum squared distance (MSD) of the 4-D co-set is 4d20 and the branch minimum squared distance trellis tree is also 4d20. This invention is an improvement on the existing 64 state 4-D trellis encoder recommended by the ITU-T (international telecommunication union).
Since the Euclidean distance between symbols is a paramount feature of the QAM signal, and since the discrete multi-tone modulation scheme employed by some of the high data rate systems such as DSL requires transmitting of at least three DMT symbols per bin, there is a need to use Trellis codes in DMT.